Biochemical and biophysical comparison of two mucins from human submandibular-sublingual saliva

A high-molecular-weight mucin-glycoprotein (MG1) was isolated from human submandibular-sublingual saliva and was comprised of 14.9% protein, 29.0% N-acetylglucosamine, 9.4% N-acetylgalactosamine, 10.5% fucose, 24.2% galactose, 0.9% mannose, 4.0% N-acetylneuraminic acid, and 7.0% sulfate. Carbohydrate units were O-glycosidically linked and ranged in size from 4 to 16 residues. The biophysical properties of MG1 were compared to those of a smaller mucin (MG2) also isolated from submandibular-sublingual saliva. Fluorescence spectroscopy demonstrated that MG1 bound both 1-anilino-8-naphthalenesulfonate (ANS) and N-phenyl-1-naphthylamine (NPNA) in stable hydrophobic binding sites (melting temperature, 47 +/- 2 degrees C), whereas MG2 did not bind these hydrophobic probes. These hydrophobic domains occurred on nonglycosylated or naked portions of MG1 since Pronase treatment eliminated ANS binding. Reduction of disulfide bridges in MG1 increased the number of available hydrophobic binding sites. High ionic strength (0 to 2 M NaCl) had no effect on ligand binding, whereas lowering pH (9 to 2) increased ANS binding without affecting NPNA complexation. Circular dichroism (CD) data suggested that MG1's carbohydrate chains dominated its spectrum. In contrast, the peptide backbone dominated the CD spectrum of MG2. Collectively, the results of this study indicate that human submandibular-sublingual saliva contains two structurally distinct mucins.